Current work vehicles, such as tractors and other agricultural vehicles, include an electronically controlled engine and a transmission, such as a power shift transmission (PST) or a continuously variable transmission (CVT) coupled to the engine. The transmission is, in turn, coupled to at least one drive axle assembly for transferring torque from the transmission to the vehicle's wheels. As is generally understood, each drive axle assembly may include one or more bearings and/or gears to assist in transmitting torque between the transmission and the drive wheels.
By including an electronically controlled engine within a work vehicle, the torque available from the engine may be controlled accurately. As a result, manufacturers have attempted to protect certain components of the work vehicle by defining specific torque limits for the engine, which in turn, provides corresponding output torque limits for the transmission. Currently, such engine/transmission torque limits have been implemented by defining a fixed torque limit for each gear of the transmission.
For example, FIG. 1 illustrates a chart providing example operating curves and torque limits for a plurality of different transmission gears (for ease of illustration, the example data is only provided for the first, second, third, fifth and seventh transmission gears). As shown, for each gear, a maximum operating curve is defined that corresponds to the maximum transmission output torque and speed combinations that may be achieved for each gear based on the operating limits of the engine (e.g., at the maximum speed and torque limits for the engine). Specifically, the first gear is associated with a first maximum operating curve 302, the second gear is associated with a second maximum operating curve 304, the third gear is associated with a third maximum operating curve 306, the fifth gear is associated with a fifth maximum operating curve 308 and the seventh gear is associated with a seventh maximum operating curve 310. In addition, a fixed engine torque limit is defined for each gear, which, in turn, provides fixed transmission output torque limits for the various gears. For example, as shown in FIG. 1, a first transmission torque limit 312 is defined for the first gear, a second transmission torque limit 314 is defined for the second gear, a third transmission torque limit 316 is defined for the third gear, a fifth transmission torque limit 318 is defined for the fifth gear and a seventh transmission torque limit 320 is defined for the seventh gear. Of course, maximum operating curves and corresponding fixed torque limits may also be defined for any of the other gears of the transmission.
By using such fixed torque limits, an operator may, in certain instances, be provided with a vehicle performance that is less than desirable. For example, when the engine lugs and loses speed as a result of increased vehicle loading, the transmission output speed is also reduced. However, due to the fixed torque limits, the transmission output torque is capped at the corresponding torque limit as the output speed is reduced. This results in an undesirable performance “feel” for the operator and often leads to the operator having to downshift to allow for an increase in engine/transmission torque. In addition to the impact on the overall performance of the work vehicle, the individual torque limits also increase the complexity of developing suitable vehicle control systems. For example, each work vehicle may require a plurality of different torque limits (e.g., one for each gear). Moreover, such torque limits may vary significantly across different vehicle configurations based on the specific capabilities of the components included within each vehicle and the loading requirements for each vehicle.
Accordingly, a system and method that simplifies the manner in which the transmission output torque is limited and that also improves the overall performance “feel” for the operator would be welcomed in the technology.